I just booked train tickets from London to Liverpool August 24th. When I try to book return tickets on August 27th it shows lots of choice but it says "no fare available" next to each of them. Why?
Avanti is fraught with issues so doesn't sell as early as others, especially weekends.
They estimate to release the schedule on 5th July for that date.
There is some planned engineering work at Euston station on that weekend, which may well affect your journey. As yet the full details are not finalised but they will be nearer the date. I expect there will be ways of doing the journey but it might take a bit longer and there may well not be cheaper Advance tickets.
August 27 is also the Sunday of a bank holiday weekend.
It's pretty certain that Euston will be open that weekend, so you won't end up on a creative route back, maybe involving a coach. If only because Kings Cross is closed, and there is an industry rule that either east or west coast should always be open to London.
More often than not when Avanti give a date for timetable release it slips by a day or two- not their fault, its due to Network Rail. 5 July is still close to 8 weeks which is the norm now for engineering timetable release.